On the platform of Koku Koen station on the Seibu Shinjuku Line. They have flowers on the platform fence. They also have small benches to sit on while you wait for the train, like this girl is doing.
Seibu Railway used this spot for their promotional poster this month (see below). I think they're trying to make the station more friendly by putting plants in and around the Seibu Line stations. Great idea.